(1) Field of the Invention
The present invention is a system and method of use for detection of surface vessels from underwater.
(2) Description of the Prior Art
Surface wakes have been detected using electromagnetic radiation spanning microwave and visible wavelengths. A surface sea state and wake bubble content can provide wake features that are observable under solar illumination. However, underwater detection of the solar illumination is complicated by absorption and light scattering in water. Underwater LIDAR systems operating at blue/green wavelengths have been used to detect wakes by detection of laser pulse back scattered light from wake bubbles.
For microwave frequencies, water absorption is of a magnitude that only above surface detection is possible. Above surface detection of wakes at microwave frequencies use radar that produce signal returns that depend on surface wave height and geometry including an average return from capillary waves over an illuminating microwave beam.
In the art, Lubard et al. (U.S. Pat. No. 6,836,285) discloses a specular return, called “glints”, which can vary over orders of magnitude and can be either much larger or much smaller than a volume backscatter return. The difficulty that these different returns cause in estimating the surface position is that the volume back scattered return reaches a peak value only after the laser pulse is entirely within the water volume, while the glint signal reaches a peak when the peak of the pulse arrives at the water surface. The distance between these peaks is based on the laser width.
In Slater (U.S. Pat. No. 7,551,519), a sample block diagram depicts a feed-forward filter element wherein the scintillation corrupted gain reference signal is the DC and subsonic signal components from the photosensitive element. The in-band signal is normalized (divided) by the remote optical receiver link gain estimate derived from the DC carrier and subsonic signal components that significantly reduce unwanted amplitude modulation induced by atmospheric turbulence along a line of sight between a passive long range acoustic sensor and the glint associated with an acousto-optical modulator or associated wake turbulence.
In Lubard et al. (U.S. Pat. No. 7,683,928), two different returns estimate a surface position with a volume backscatter return reaching a peak value only after the laser pulse is entirely within the water volume. The glint surface reaches a peak when the peak of the pulse arrives at the water surface. This data has a relatively coarse resolution.
In Lubard et al. (U.S. Pat. No. 7,688,348), a resultant surface mapping algorithm is applied to existing Streak Tube Imaging Lidar (STIL) data taken during other tests. Working with this data provides insight into the discrimination of volume back scattered from glint.
In Mullen et al. (U.S. Pat. No. 8,044,999), pulsed laser sources are used in underwater laser-imaging systems to temporally discriminate against scattered light and to provide object range information. A typical configuration is broad-beam illumination of the scene and a gated intensified camera receiver—although systems using photomultiplier tube receivers in both single and multiple pixel configurations have also been utilized. The Streak Tube Imaging Lidar (STIL) uses a pulsed laser transmitter in a scanner-less configuration.
Williams (U.S. Pat. No. 8,207,484) discloses that the intensity of the return light received by a sensor channel in a LIDAR system used for detection of submerged objects, varies over a comparatively wide range. It has been estimated that under some conditions, a specular reflection (glint) from the water surface might deliver as many as seven times the photons to a single sensor channel whereas the return signal due to back-scatter could be eight orders of magnitude smaller. Each channel of the streak camera includes a glint detector that receives an output signal of a preamplifier. Each channel asserts an output in a high state in response to detection of an input signal of sufficient magnitude to be associated with a specular reflection from the water surface.
In Mead et al. (U.S. Pat. No. 8,953,647), a block diagram depicts a visible-light sensing/imaging system using a frequency-quadrupled IR laser transmitter system. The system outputs a pulsed waveform (amplitude modulated) transmitted laser beam that detects light in the same narrowband wavelength range and processes the received reflections (which are water-object-interaction to water-detector light signals) to generate two-dimensional and/or three-dimensional image information which is output.
In Shiba (United States Patent Application No. 2012/0242533), an object detection support device is disclosed which supports the detection of an object by deflected waves generated from a transmitter. The transmitter is one part of a device that detects and tracks the object, such as a sonar system, radar system or a LIDAR system used to search, detect or range the object in the water.
Based on the success of existing underwater LIDAR systems, there would be a substantive advantage to make use of strong surface glints observed by an innovative system that can focus and manage an output light pulse onto the water surface.